The present invention generally relates to methods and apparatus for cutting, cleaving, or forming waveguides to precise differential lengths. In particular, the present invention relates to methods and apparatus for cleaving two or more optical waveguides to precise differential lengths.
There are numerous applications in fields such as communications, testing, and measurement that require two or more waveguides having precise differential lengths. In particular, there are many applications in these fields that require two or more optical fibers having precise differential lengths.
Prior art methods of cleaving optical fibers to precise differential lengths have limited accuracy and have numerous other disadvantages. For example, one prior art method involves repeatedly polishing the end face of the optical fiber and then measuring its optical path length. In this method, the optical fiber is cut to an approximate length that is greater than the desired length and the optical path length of the fiber is measured. The end face of the optical fiber is then polished to reduce the length of the optical fiber and the new optical path length of the fiber is measured. This process of polishing and measuring is repeated until of the desired fiber length is achieved. This prior art method is very time and labor intensive and, therefore, is not suitable for manufacturing components in large volume. In addition, the polishing may damage the optical fiber making it unsuitable for some applications.
Another prior art method of cleaving optical fibers to precise differential lengths uses thermal fiber stretching techniques to change the length of the fiber to the desired length. This prior art method uses a fusion splicer or optical fiber furnace to heat and stretch the optical fiber to the desired length. This prior art method also has numerous disadvantages. The equipment for thermally stretching and cutting optical fibers to precise lengths is expensive, physically large and complex.
In addition, optical fibers can only be stretched in a limited range and stretching an optical fiber may weaken the fiber and, therefore, make it susceptible to failing. Therefore, thermal fiber stretching methods may not be suitable for optical fiber systems that require high reliability. Also, stretching an optical fiber may change the polarization and dispersion properties of the optical fiber. Therefore, thermal fiber stretching techniques may not be suitable for some applications and for some special types of optical fiber, such as polarization maintaining and dispersion compensating optical fiber.
The present invention relates to cutting, cleaving or forming waveguides to precise differential lengths. By differential length we mean the difference in length from one waveguide to another waveguide. The methods and apparatus of the present invention apply to any type of waveguide including high frequency and optical waveguides.
It is an object of the present invention to provide a method for cleaving two or more optical fibers to an accuracy on the order of 100 microns or less. It is another object of the present invention to improve the manufacturability of components for high-speed optical time-domain multiplexing (OTDM) communication systems, such as OTDM multiplexers and demultiplexers. It is another object of the present invention to improve the manufacturability of components for differential detection systems including coherent detection systems.
A discovery of the present invention is that two or more optical waveguides can be cut to a differential accuracy of less than 20 microns by aligning a cleaving tool at a position that is determined with reference to two optical time-domain reflectometry (OTDR) measurements. In one embodiment of the invention, one OTDR measurement is taken to an end of the waveguide and the other OTDR measurement is taken to a reference mirror positioned in the path of radiation propagating from the end of the waveguide.
Accordingly, the present invention features a method of cutting waveguides to precise differential lengths. The method includes coupling a first end of a waveguide to an input port of a reflectometer. A reference mirror is then positioned in a path of radiation propagating through the second end of the waveguide. A waveguide cutting tool is then positioned proximate to the waveguide and at a distance relative to a reference mirror.
A first reflectometry measurement is performed on the waveguide to a second end of the waveguide. A second reflectometry measurement is performed on the waveguide to the reference mirror. The first and/or second reflectometry measurements may be OTDR measurements. The waveguide is then positioned relative to the reference mirror and waveguide cutting tool so that the first reflectometery measurement is a measurement increment apart from the second reflectometry measurement. The measurement increment may be a time or a distance measurement increment. The waveguide is then cut with the cutting tool positioned at the distance relative to the reference mirror.
The above method is repeated for a new waveguide with the measurement increment being the same measurement increment displaced by a second measurement increment that corresponds to a desired differential length between the waveguide and the new waveguide. The same measurement increment may be displaced by the second measurement increment by adding the measurement increment to the second measurement increment or by subtracting the measurement increment from the second measurement increment.
The present invention also features a method of cutting two optical fibers to a precise differential length. The method includes coupling a first end of a first optical fiber to an input port of a reflectometer. A reference mirror is then positioned in a path of radiation propagating through the second end of the first optical fiber. A fiber cleaving tool is then positioned proximate to the first optical fiber and at a distance relative to a reference mirror.
A first reflectometry measurement is performed on the first optical fiber to a second end of the first optical fiber. A second reflectometry measurement is performed on the first optical fiber to the reference mirror. The first and/or second reflectometry measurements may be OTDR measurements. The first optical fiber is then positioned relative to the reference mirror and fiber cleaving tool so that the first reflectometery measurement is a measurement increment apart from the second reflectometry measurement. The measurement increment may be a time or a distance measurement increment. The first optical fiber is then cut with the cutting tool positioned at the distance relative to the reference mirror.
The above method is repeated for the second optical fiber with the measurement increment being the same measurement increment displaced by a second measurement increment that corresponds to a desired differential length between the first optical fiber and the second optical fiber. The same measurement increment may be displaced by the second measurement increment by adding the measurement increment to the second measurement increment or by subtracting the measurement increment from the second measurement increment.
The present invention also features a method of manufacturing a bit interleaved optical multiplexer having N channels. The method includes cleaving N output optical fibers of a 1xc3x97N optical splitter to desired differential lengths. Each of the N output optical fibers is cleaved by aligning an optical fiber cleaving tool at a position along the output optical fiber that is determined with reference to two OTDR measurements of the output optical fiber. In one embodiment, the two OTDR measurements of the output optical fiber comprise a first OTDR measurement to an end of the output optical fiber and a second OTDR measurement to a reference mirror.
N fiber pigtail modulator sections are then cleaved to desired differential lengths. Each of the N fiber pigtail modulator sections is cleaved by aligning an optical fiber cleaving tool at a position along the pigtail modulator section that is determined with reference to two OTDR measurement of the pigtail modulator section. In one embodiment, the two OTDR measurements of the input optical fiber comprise a first OTDR measurement to an end of the input optical fiber and a second OTDR measurement to a reference mirror.
N input optical fibers of a 1xc3x97N optical combiner are then cleaved to desired differential lengths. Each of the N input optical fibers are cleaved by aligning an optical fiber cleaving tool at a position along the input optical fiber that is determined with reference to two OTDR measurement of the input optical fiber. In one embodiment, the two OTDR measurements of the pigtail modulator section comprise a first OTDR measurement to an end of the pigtail modulator section and a second OTDR measurement to a reference mirror.
A respective one of the N channels of each of the bit interleaved optical multiplexer has a desired optical path length that includes a combination of a respective one of the optical path lengths of the N output optical fibers, a respective one of the N fiber pigtail modulator sections, and a respective one of the N input optical fibers. In one embodiment, at least two of the desired differential lengths of the N output optical fibers of a 1xc3x97N optical splitter are substantially zero. In one embodiment, at least two of the desired differential lengths of the N input optical fibers of a 1xc3x97N optical combiner are substantially zero.
The present invention also features an optical fiber cleaving apparatus for cleaving optical fibers to precise differential lengths. The optical fiber cleaving apparatus includes a reflectometer that has an input port that accepts a first end of an optical fiber to be cleaved. In one embodiment, the reflectometer is an OTDR reflectometer. A reference mirror is positioned in the path of an optical beam propagating from a second end of the optical fiber. In one embodiment, the reference mirror is moveable and may be mounted on a precision translation stage. In one embodiment, the optical fiber is mounted on a precision translation stage and the position of the optical fiber relative to at least one of the fiber cleaving tool and the reference mirror is determined by the precision translation stage.
A fiber cleaving tool is positioned at a distance relative to the reference mirror. The fiber cleaving tool cleaves the optical fiber at a desired position that is determined with reference to a first reflectometry measurement to the end of the optical fiber and a second reflectometry measurement to the reference mirror.